I just bought what WAS a rental property in Missouri. I had told the renters of the sale and given them until the 14th of February to move. I was told by the tenant that I had to GET OUT of her home, she knew her rights.
I am needing to move into this place, because I am going to be the person living there, I did not purchase this place to be a rental property.
What can I do?

When did you close on the home? Is there a lease in effect? How much notice did you give the tenants?

The tenants do have rights. When you bought the home you inherited the lease and you have to abide by the terms of it. So the first thing you need to do is get a copy of the lease from the previous owner. You cannot get them out until the lease expires. IF they are on a month to month lease, then you have to give them the amount of notice of termination of the lease prescribed by the lease. If the original lease said 60 days then you have to give 60 days. If no time limit was specified then you have to give them a full rental period. So if rent is due on the first, you would have to give them until 4/1 at this point.

Then, if they still refuse to move you would have to evict them.

I wonder where you got the idea you could buy a tenanted rental property and just throw the renters out with a few days notice.

Yes, as Scott said, once you close on the home, you also inherit the lease or rental agreement, if there is a written lease for any period you will have to honor it, If their rental is month to month you will have to give them at least a 30 day notice to move.

And yes, she had the right to throw you out for trying to violate her rights, you need to tell you you are sorry, since you can not throw them out without legal notice according to their rental agreement,